This disclosure pertains to optical spectrometers in general and in particular to an optical spectrometer such as an Offner-Chrisp-type spectrometer using wide field of view imaging fore-optics such as Wide Angle Large Reflective Unobstructed System (WALRUS)-type fore-optics.
Offner-Chrisp spectrometers are extensively used as tools for hyperspectral imaging. The spectrometers allow capture of the spectral signature of an object and provide imaging capabilities with low chromatic aberrations in a relatively compact configuration. The Offner-Chrisp spectrometer has three spherical concentric or near-concentric elements: two concave mirrors and one convex reflective grating. A detailed description of such a spectrometer can be found in U.S. Pat. No. 5,880,834 to Michael P. Chrisp entitled “Convex Diffraction Grating Imaging Spectrometer,” the entire contents of which are incorporated herein by reference.
FIG. 1 depicts a conventional Offner-Chrisp spectrometer. Offner-Chrisp Spectrometer 10 includes concave spherical mirrors M1 and M2 and spherical convex reflective diffraction grating G. The spherical mirrors M1/M2 are positioned opposite the grating G. The spherical mirror M1 has a radius R1, and the spherical mirror M2 has a spherical radius R2. Radii R1 and R2 can be equal or different depending on the desired configuration. Offner-Chrisp spectrometer 10 may further include a radiation detector such as a Focal Plane Array (FPA) 12 and entrance slit 14 provided in housing 15 of spectrometer 10 for allowing radiation to enter spectrometer 10. First concave mirror M1 is oriented to receive rays entering through entrance slit 14. The rays reflected by first concave mirror M1 are directed towards grating G. Grating G includes grooves for diffracting incident radiation and dispersing, i.e., resolving the incident radiation into wavelength spectral components. Second concave mirror M2 is oriented to receive the diffracted wavelength radiation component from grating G and to reflect the wavelength radiation components onto focal plane array FPA 12 for detection. Offner-Chrisp spectrometer 10 typically operates in a “pushbroom” configuration where the spectrometer moves in a direction substantially perpendicular to entrance slit 14 to cover or scan a surface of the object.
In order to be able to use an Offner-Chrisp spectrometer in applications such as, for example, remote sensing (e.g., terrestrial or other planetary and space remote sensing), the Offner-Chrisp spectrometer has to be coupled to fore-optics. The fore-optics are configured to receive or capture radiation from a distant object and form an image at the entrance slit of the Offner-Chrisp spectrometer, e.g., entrance slit 14 of Offner-Chrisp spectrometer 10. An example of such optical system combining the use of fore-optics with the use of an Offner-Chrisp-type spectrometer is disclosed in U.S. Pat. No. 6,100,974 to Francis M. Reininger, entitled “Imaging Spectrometer/Camera Having Convex Grating,” the entire contents of which are incorporated herein by reference. U.S. Pat. No. 6,100,974 discloses coupling Wetherell-type reflective triplet fore-optics to an Offner-Chrisp-type spectrometer. A detailed description of Wetherell-type fore-optics can be found in U.S. Pat. No. 4,240,707 to Wetherell et al., entitled “All-Reflective Three Element Objective,” the entire contents of which are incorporated herein by reference.
FIG. 2 depicts an example of conventional Wetherell-type fore-optics. Wetherell-type fore-optics 20 comprise positive power concave primary mirror 21, negative power convex secondary mirror 22, and positive power concave tertiary mirror 23. Secondary convex mirror 22 is positioned between and opposite primary concave mirror 21 and tertiary concave mirror 23. In operation, radiation emanating from a distant object (represented by radiation beams 25 and 25′) impinges on positive power primary mirror 21 which reflects the radiation towards negative power secondary mirror 22. Negative power secondary mirror 22 is positioned and arranged to receive radiation reflected by the positive power primary mirror 21 and to reflect the radiation towards positive power tertiary mirror 23. Positive power tertiary mirror 23, in turn, directs the radiation towards radiation detector 24, or as in the case of the optical system in Reininger, directs the radiation towards the entrance slit (e.g. slit 14 in FIG. 1) of the Offner-Chrisp-type spectrometer for spectral analysis of the radiation.
Conventional Wetherell-type reflective triplet fore-optics provide only a limited field of view (FOV). As a result, the field of view that can be spectrally analyzed by an Offner-Chrisp-type spectrometer may be limited when the fore-optics coupled to the Offner-Chrisp-type spectrometer is Wetherell-type fore-optics.
Therefore, there is a need in the art for fore-optics that can provide a wide field of view (WFOV) that can be coupled to a spectrometer, such as an Offner-Chrisp-type spectrometer, so as to be able, for example, to obtain a wavelength spectrum of a wider area of an object (e.g., to obtain a wavelength spectrum of a wider portion of the surface of the earth, etc.). In this context, a “wide” FOV may be considered, by way of a non-limiting example, to be greater than 20 deg.